The present invention relates to an electrical discharge apparatus for machining at least one rectilinear cooling air duct having a flared exhaust orifice in the wall of a hollow blade of a gas turbine engine. The apparatus includes at least one electrode having a given geometry and mounted on a first electrode support arranged to machine the flared exhaust orifice, and means for driving the motion of the electrode support. The electrode comprises a rear end segment that is affixed to the first electrode support.
The output of aircraft gas turbines increases continuously. Illustratively, engine thrust increases with the temperature at an intake position of an engine turbine.
Raising this temperature, however, is limited by the heat resistance of the turbine""s guide vanes and rotors since the strength of their material decreases as the temperature rises.
It is known to cool the outer blade wall with cooling air that is tapped into the compressors"" zone and circulates in inner cavities defined in the blades that is evacuated through ducts fitted into the outer walls of the blades. The exhaust orifices of the ducts are widened and flared in order for the cooling air to constitute a cooling film that flows along the blade wall.
It is known to machine these air ducts by electrical discharge. However, one electrode is simultaneously used to manufacture the duct""s flared orifice and the duct itself. As a result, the electrode geometry is complex and machining an electrode is difficult.
Moreover, one air duct is made at a time, and as a result, the electrode must be adjusted relative to the blade between each machining operation, entailing a substantial increase in manufacturing time.
The objective of the present invention is to provide tooling that reduces the costs of machining air ducts by using a set of interchangeable electrodes of simple geometries.
The present invention attains its goal by providing a forming electrode that comprises a first side fitted with a groove for guiding a straight electrode that machines a straight portion of the air duct, and furthermore, a middle segment of constant cross-section and a pointed front end segment of which the cross-section decreases from the middle segment to the tip of the forming electrode.
Advantageously the front end segment, in addition to the first side, also comprises three beveled facets.
The facets may slope at different angles, thereby making it possible to define a particular geometry for any kind of flared orifice.
Preferably, the cross-section of the middle segment of the forming electrode is quadrangular. In fact, the forming electrode is advantageously made from a cross-sectionally rectangular bar and the groove has a V-shaped cross-section.
The forming electrode is merely made of a cross-sectionally quadrangular bar into which is fitted a longitudinal groove, for instance of V-shaped cross-section, the end of the electrode tapering to constitute a tip element.
Advantageously, the apparatus of the invention also comprises a second electrode support supporting at least one straight electrode made to slide in the groove.
By providing the apparatus of the invention with two electrodes, easily machined electrodes may be used. Also, adjusting the straight electrode relative to the forming electrode makes it possible to define different geometries for the ducts to be machined. Again, in the event of wear, the defective electrode may be replaced while retaining the other.
Preferably, the straight electrode is cylindrical. As a result, if the second electrode is worn, it may be easily replaced.
Preferably, the apparatus of the invention also includes a plate to keep the cylindrical electrode in the groove.
In general, the electrodes are long relative to their cross-section and accordingly they become more susceptible to breaking the farther the electrode support is from the machining site. The plate acts as a cover, which together with the electrode""s V-shape, defines a housing for the cylindrical electrode, the latter thereby being guided when in the vicinity of the zone where the air ducts are being machined.
Preferably, the apparatus of the present invention is fitted with means simultaneously moving the first and the second electrode supports.
The two electrode supports are displaced jointly, the cylindrical electrode positioned to the rear of the forming electrode in order to first machine the flared orifice of the air duct.
Preferably, the apparatus of the invention also is fitted with means to displace the first electrode support and the second electrode support independently of each other.
The moment the air duct""s flared orifice has been completed, the cylindrical electrode continues its motion until the cylindrical duct has been completed. In this manner, the stresses applied to the electrodes during machining are spread out better.
Preferably, the first electrode support holds a plurality of forming electrodes and the second electrode support holds a plurality of straight electrodes.
Because the electrodes may be adjusted mutually during one and the same operational stage, a set of air ducts can be machined simultaneously, preferably in one and the same blade.